1. Field of the Invention
The present invention relates to a heat treatment for producing an iron powder that is directly used in the form of a powder or is used for powder metallurgy. In particular, the present invention relates to a finish heat treatment method for an iron powder in which a product iron powder is obtained by subjecting a raw iron powder to at least two treatments selected from decarburization, deoxidation, and denitrification, and to a finish heat treatment apparatus used in the method.
2. Description of the Related Art
A raw iron powder such as a rough-reduced iron powder obtained by rough-reducing a mill scale or an as-atomized iron powder has been conventionally subjected to a finish heat treatment to obtain a product iron powder. In the finish heat treatment, at least one treatment selected from decarburization, deoxidation, and denitrification is performed on the raw iron powder in accordance with the applications of the product iron powder. Normally, the finish heat treatment is continuously performed using a moving hearth furnace.
For example, Japanese Unexamined Patent Application Publication No. 52-156714 (Patent Document 1) discloses a method for heat-treating a raw material iron powder in which, when a raw material iron powder is subjected to a continuous heat treatment in an ambient gas mainly composed of a hydrogen gas in order to obtain a reduced iron powder, the ambient temperature of the heat treatment is kept at 800 to 950° C., the heat treatment in the first half is performed in a decarburizing atmosphere having a water content of 6% or more by volume, and the heat treatment in the second half is performed in a reducing atmosphere having a water content of 4% or less by volume.
Japanese Examined Patent Application Publication No. 01-40881 (Patent Document 2) discloses a continuous moving hearth furnace in which a moving hearth furnace is partitioned into a plurality of spaces with partition walls that are disposed in a direction perpendicular to the raw material moving direction; a gas passageway is formed in the partitioned spaces so that a gas flows in a direction opposite to the moving direction of the moving hearth; and a gas stirring apparatus is disposed on the upper portion of each of the spaces. In the technology disclosed in Patent Document 2, with this continuous moving hearth furnace, a finish heat treatment is performed on a steel powder by continuously performing two or more treatments selected from decarburization, deoxidation, and denitrification. In this technology, the treatments of the decarburization, the deoxidation, and the denitrification are independently performed in the partitioned spaces of the moving hearth furnace. The temperatures of these treatments are independently controlled to 600 to 1100° C. in the decarburization, 700 to 1100° C. in the deoxidation, and 450 to 750° C. in the denitrification.
FIG. 2 shows a finish heat treatment apparatus of the same type as the continuous moving hearth furnace disclosed in Patent Document 2. The finish heat treatment apparatus shown in FIG. 2 includes a furnace body 30 partitioned with partition walls 1 into a plurality of zones, that is, a decarburization zone 2, a deoxidation zone 3, and a denitrification zone 4, a hopper 8 disposed on the entry side of the furnace body 30, wheels 10 disposed on the entry side and exit side of the furnace body 30, a belt 9 that is continuously rotated by the wheels 10 and moves around each of the zones of the furnace body 30, and radiant tubes 11. A raw iron powder 7 supplied from the hopper 8 onto the belt 9 that continuously moves due to the continuous rotation of the wheels 10 is heat-treated while moving in the zones 2, 3, and 4 that are heated to proper temperatures with the radiant tubes 11. As a result, the raw iron powder 7 is subjected to decarburization, deoxidation, and denitrification and thus a product iron powder 71 is obtained. In the technology disclosed in Patent Document 2, the reaction in each of the zones is believed to be as follows.
In the decarburization zone 2, the decarburization of the raw powder is performed by controlling the ambient temperature to 600 to 1100° C. using the radiant tubes 11 and by controlling the dew point of the ambient to 30 to 60° C. by adding water vapor (H2O gas) introduced from a water vapor blowing inlet 12 disposed on the downstream side of the decarburization zone 2 to an ambient gas sent from the deoxidation zone 3. An ambient gas outlet 6 is disposed on the upstream side of the decarburization zone 2 and thus the ambient gas is released to the outside of the apparatus.
In the deoxidation zone 3, the deoxidation of the raw powder is performed by controlling the ambient temperature to 700 to 1100° C. using the radiant tubes 11 and by providing an ambient gas (a hydrogen gas having a dew point of 40° C. or less) sent from the denitrification zone 4.
In the denitrification zone 4, the denitrification of the raw powder is performed by controlling the ambient temperature to 450 to 750° C. using the radiant tubes 11 and by introducing a hydrogen gas (dew point: 40° C. or less), which is a reactant gas, from an ambient gas inlet 5 disposed on the downstream side of this denitrification zone 4.